The present invention relates to a method for estimating connection quality in a receiver of a radio system, the method comprising decoding a received signal with a Viterbi decoder by utilizing a trellis diagram comprising a group of state points, a transition being possible from each point of the diagram to two points in the next column, and state transition values being calculated for the transitions.
In a cellular radio system, the quality of the connection between a base station and a subscriber terminal varies continuously. This variation is caused by disturbances on the radio path and the attenuation of radio waves as a function of distance. For example, when a subscriber terminal moves further away from the base station, attenuation on the connection between the subscriber terminal and the base station increases. Often, an attempt is made to compensate for this attenuation by power control.
Power measurement alone will not, however, suffice as a parameter indicating the connection quality. A variable often employed in digital cellular radio systems to express connection quality is the bit error rate (BER), which represents the number of erroneously received bits compared to all the received bits. In the GSM cellular radio system, for example, the base station and the subscriber terminal are continuously measuring the BER on the connection between them. The measurement results are transferred to a base station controller, which on the basis of the measurements makes, if need be, the decision to hand the connection over to a channel offering better connection quality.
It is necessary to measure the connection quality as accurately as possible so as to be able to use the resources of the cellular radio system optimally and to minimize the number of poor connections. Consequently, the measurement period for the BER measurement in the GSM system is 480 milliseconds, which corresponds to 104 TDMA timeslots in a full-rate channel and 52 TDMA timeslots in a half-rate channel.
It is previously known to estimate the BER of a channel as a so-called pseudober measurement. In this method, the BER is estimated by coding anew the signal at the output of the channel decoder comprised by the receiver, and by counting the number of correct bits. However, this method is not the best possible because it is not guaranteed that the channel decoder always corrects an erroneous bit. The channel decoder may also change a correct bit into an incorrect bit. In addition, the accuracy of the method depends on the length of the bit sequence being treated. The bit sequence, i.e. the measurement period, should be long enough so that the BER could be detected correctly also when erroneous bits occur seldom in the channel. In the GSM system, this is a problem particularly in a half-rate channel, in which the number of bits is lower than in a full-rate channel. Further, the pseudober method is heavy to carry out by a signal processor.
Another prior art method for estimating the BER is to calculate it from the known training sequence of each traffic channel during the measurement period. Since in the GSM system there are 26 bits in the training sequence, and a half-rate channel has 52 frames on each traffic channel during the measurement period, there are consequently only 52*26 known bits from which to calculate the BER. This is too few for calculating a BER estimate accurately enough.
It is therefore an object of the invention to implement a method for estimating the quality of a connection, which is simple and accurate and does not require heavy computation. It is a further object of the invention to implement a receiver in which monitoring of the connection quality is easy to implement in connection with the decoding process.
This is achieved by a method of the type set forth in the introduction, characterized by calculating, in each column of the trellis diagram, the maximum values for both the transitions of the points in the column, and by determining the absolute value of the difference of the calculated maximum values, and by utilizing said absolute value in determining the connection quality.
The invention further relates to a receiver in a radio system, the receiver comprising means for decoding a received signal by the Viterbi method by means of a trellis diagram comprising a group of state points, and means for calculating state transition values for the two possible transitions in each point of the diagram. The receiver according to the invention is characterized by comprising means for calculating, in each column of the trellis diagram, the maximum values for both the transitions of the points in the column, for determining the absolute value of the difference of the calculated maximum values, and means for estimating connection quality by utilizing said absolute value.
The inventive solution provides a number of advantages. It may be implemented in a simple manner, e.g. by an ASIC circuit, and it does not require memory. When implemented in connection with Viterbi logic, the necessary computation may be executed in parallel with carrying out the Viterbi. Further, the inventive method is even more accurate than the previously known methods.